Oh, to return to the days of the Golden Age of the Broadway musical in the '40s and '50s and '60s when future classics first got their acts together in out-of-town places like New Haven's Shubert Theatre.

To have been at opening night for musicals like "Oklahoma!" (when it was first called "Away We Go!"), or "The King and I" or "The Sound of Music." Or better yet, to witness the frantic backstage dramas, like when a terrified Rex Harrison refused to go on stage as theatergoers made their way to the theater in the middle of a snowstorm for the world premiere of "My Fair Lady."

Those days are evoked in the new musical "Seen Change!" by New Haven's A Broken Umbrella Theatre that celebrates and marks the milestones of 100 years of theater at the Shubert. Performances run Feb. 18 to 28.

The show takes place in and around the Shubert Theatre on College Street in downtown New Haven with audience members following the action — first in the lobby of the theater, then moving to the adjacent lobby of the former Taft Hotel — where creative teams in the day burned the midnight oil reworking their shows — and returning to the inside of the theater itself.

Here's the fictional premise, according to ensemble member Rachel Alderman:

"Back in 1944 there was this very famous composer-lyricist named Timothy T. Willoughby who was bigger than Rodgers & Hammerstein, bigger than Sondheim, bigger than them all and he had many premieres at the Shubert. But in the making of this show — "My Heart Is In Your Hands" — something happened and it never went on and never made it to Broadway.

"Jump to 2015 and there's a new team that's revising that show at the Shubert to make it [the 1944 musical] what it should have been," she says.

"Our 'Seen Change!' audience plays the part of an invited group of backers who have been supporting this revival and who have been invited for this dress rehearsal the night before the opening."

The "backers"/audience — there's a limit of 214 people who can participate — are being told what they are about to see as they prepare to go into the theater.

But then a crisis occurs when an apprentice at the Shubert inadvertently defies an age-old theater superstition and the action then moves to the lobby of the former Taft Hotel before it returns to the main hall of the theater where most of the rest of "Seen Change!" takes place. (While the lobby action is free flowing, the final action of "Seen Change!" has specific ticketed seat locations for theater-goers in the orchestra section of the Shubert.)

"It's literally a journey," says Alderman, of the 20-member cast show in which two theatrical eras collide.

While the newly renovated lobby of the Shubert theater is very different from decades past, the spiffed up orchestra and balconies look very much the same as they did during the theater's mid-century heyday. The Taft, which is now a condominium building, still has the grand lobby it had in that earlier era, too.

The musical-within-the musical is based on a number of shows from the Schubert's golden era, with nods to those tuneful classics. "There's also a kind of Moppet atmosphere too," says Alderman, "with this crazy ragtag group of people trying to get a show on in its feet. There are no puppets but it's definitely living in that world."

Thematically, the show is "about the passion and struggle of creating a big piece of musical theater and his many hands has to make that happen. And no matter what, the curtain will go up."

Alderman says the Schubert — which is celebrating its 100th anniversary this season — and the Taft condo "were incredible partners. Everyone understood and revered the historic and powerful dynamic between the two and wanted to celebrate it too. We got lucky."

The local theater company, which received a special award last year from the Connecticut Critics Circle, has built a reputation for taking themes, characters and stories from the community and crafting theatrical works, often in locations associated with the subject matter.

Over the past 10 years the company produced "Gilbert the Great," about the local inventor of the Erector set in the factory where it was first manufactured; the musical "Freewheelers," which tapped into the history of the bicycle, and foundation garments in New Haven was set in the basement of a long-closed department store. "Play with Matches," set in the boiler room of a factory, was based on the life of 19th-century New Haven inventor Ebenezer Beecher, who developed the automated matchstick machine.

Alderman says for "Seen Change!" the company did not have to do any physical transformations of the venue.

"It's not the same as 'Freewheelers' where we had to use 10 dumpsters to clean up the site for occupancy," she says. "Here we're taking a space that people may be familiar with but make them see it in a new way that shines light on it."

"SEEN CHANGE!"has one preview performance on Wednesday, Feb. 18, opens on Thursday, Feb. 19, and continues through Feb. 28 at the Shubert Theater, 247 College St., New Haven. Performances are Wednesdays through Fridays at 8 p.m. and Saturday, Feb. 28, at 2 and 8 p.m. Information: shubert.com and 203-562-5666 or 888-736-2663.

On Wednesday, Feb. 25, there will be a post show event at The Ordinary, (the former Tap Room at the original Taft Hotel) that was a theater hangout during the era of the show, from 10:30 p.m. On Thursday, Feb. 26, there will be a free pre-show walking tour of the theater and the former Taft Hotel with New Haven historian Colin M. CA plan. The tour starts at 6:30 p.m. in Shubert Lobby (open to public —- no tickets required).